Twin Spark Theory
Normal engines have one spark plug per cylinder. However, since decades ago, Alfa Romeo insisted on putting 2 spark plugs in each cylinder, firstly in its racers and now in its cars. As ignition takes place in two locations rather than one, this enable more efficient combustion and cleaner emission. However, besides Alfa, in the past 15 years only Mercedes and Porsche have ever applied Twin Spark design to their engines. This is mainly because of the complexity of cylinder head - it would be too difficult to put 4 valves and 2 plugs into the small cylinder head area. ( Mercedes' and Porsche's engines are 3 valves and 2 valves per cylinder respectively, so they have no such problem.) Only Alfa Romeo have applied it to 4-valve engines.
In its Original 2 valve twin spark engines, Alfa actually used the twin spark plugs not to create extra power but to cure rough running at low rpm and to improve fuel economy.
In its original guise when being develop Alfa could extract a creditable 148bhp from its two litre two valve engine. The problem was that it had rough low speed running and poor low speed economy and emissions. The main cause being the high degree of overlap being used in the cams to get the high power.
What Alfa found though was that whereas in a normal high overlap engine running at low speed unburnt fuel was tumbling out of the exhaust port, and the inherently diluted mixture of the EGR system which was being used to clean up the emissions, was difficult for a single plug to fire. Under normal conditions, a single plug could ignite an air/fuel ratio of 17:1, whereas twin plugs could ignite a mixture as lean as 20.8:1.
Not only could they allow lean mixtures via EGR giving good economy, but they could ignite it reliably, giving better, smoother low speed running and cleaner emissions.
As the modern engine continues to develop though we may see more of the twin spark plug the reasons for this and its lack of previous use are many.
Older engines tended to have long stroke/ small bore combinations
Modern engines have ever-increasing bore dimensions
Modern fuesl are becoming slower burning.
Engines are getting ever higher revving
The result is that the flame at the point of ignition needs to travel ever further is shorter and shorter durations. Bigger bores means more area for plugs and valves, multiple spark plugs seems an obvious and cheap solution to getting a fast burning charge.
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